1. Field of the Invention
The present invention relates to a back pressure adjustment apparatus for a liquid ejection head which is capable of stabilizing variations in the back pressure in a liquid ejection head.
2. Description of the Related Art
A liquid ejection head has been proposed, which comprises nozzles which eject ink and pressure chambers connected to the nozzles, liquid being ejected from a nozzle by changing the pressure of the liquid inside the corresponding pressure chamber. A liquid ejection head of this kind comprises a sub tank which is connected to the liquid ejection head, and the pressure of liquid inside the sub tank is adjusted and treated as the back pressure of the liquid ejection head.
Furthermore, Japanese Patent Application Publication No. 2005-41048 and Japanese Patent Application Publication No. 2006-192785 teach the technology in which a flexible bag is provided inside a rigid container (sub tank), and by changing the volume of this flexible bag, the back pressure of a liquid ejection head connected to the bag is adjusted.
However, if the back pressure in a liquid head is adjusted by altering the volume of the flexible bag, then since the shape of the bag does not generally change in a stable fashion, sudden pressure changes may occur when the volume of the bag changes, and it is difficult to achieve fine adjustment of the back pressure.
FIG. 21A shows a state where ink has been filled into a bag 91 inside a sub tank 90 and the volume of the bag 91 is a maximum (a state where there are no creases in the bag 91), and FIG. 21B shows a state where ink has been supplied from the bag 91 in the sub tank 90 to the liquid ejection head 50 and the volume of the bag 91 has become smaller (in other words, a state where creases have occurred in the bag 91). The moment that a crease occurs, the pressure inside the bag 91 changes suddenly. Furthermore, if the volume of the bag 91 which contains creases becomes larger due to replenishment of ink into the bag 91, then the creases disappear. The moment that such a crease disappears, the pressure inside the bag 91 changes suddenly. Since sudden pressure changes occur in this way when the volume of the bag 91 varies, in general, it is difficult to achieve fine pressure adjustment at a resolution below 10 mm H2O.
Furthermore, it is known that since a bag structure is generally formed by creating joins using a thermal pressurization method, then in a state such as that shown in FIG. 21B, virtually no ink flows in the vicinity of the join regions, and aggregation of pigment or resin components (latex) arises, giving rise to ink blockages in the liquid ejection head.
Moreover, if it sought to eject ink at a resolution of 1000 dpi or above and a speed exceeding 300 mm/s, using a long head exceeding 8 inches in length, then the ink consumption per color exceeds around 20 cc/min. If ink ejection involving high duty of this kind is required, then a high-capacity bag having a volume greater than 1000 cc is required. However, since the creases described above are particularly liable to occur in the case of a large bag shape, it is generally difficult to use a bag of large capacity exceeding 500 cc.